Media curl is frequently considered one of the root causes of paper jams and registration errors during rendering, and can be exacerbated by high-density images and plural color rendering issues. Media curl can be induced by several factors such as, for example, relative humidity, paper weight, paper size, number of sides imaged, or the amount of data contained with a particular digital image.
Sheet curling can occur even in the context of unprinted sheets of paper due to changes in ambient humidity or the moisture content of the paper. Sheet curling interferes with proper sheet feeding, thereby causing sheet feeding jams, delays or registration errors. Sheet curling can cause media to come into direct contact with printing cartridges and damage the cartridges. If sheet curl is present in the output, it can interfere with proper stacking or other finishing operations. Furthermore, the amount of moisture in a sheet of paper can drastically change from the rendering process itself, which can cause or exacerbate curl.
Sheet curl problems can also occur in duplex printing, when the sheets are re-fed or re-circulated for rendering imaging material on their second sides, especially if this involves a second pass of the sheet through a thermal fuser and/or the presence of higher density images on one side than the other. The media curl must be measured and controlled so that reliable marking can be achieved and damage to the ink cartridge prevented.
Various media curl/flatness sensors and control systems are known in the electro photographic rendering arts. Such prior art systems typically employ a multiple-beam sensor such as, for example, a single cross beam sensor or a dual cross beam sensor for detecting the height/curl/flatness of the media. Such beam sensors and their precise placement with respect to the nips, transport belts, and media introduces opportunity for variability of the sensor response characteristics. For example, the nip need to be perfectly aligned with the cross beam sensor center and the media need to be shot out perfectly straight in order to achieve high measurement accuracy. Such assumptions make the manufacture and assembly errors very difficult to achieve.
Based on the foregoing, it is believed that a need exists for an improved method and system for calibrating a multiple-beam media curvature/flatness sensor to achieve high measurement accuracy without extreme requirement on the sensor manufacturing and assembling tolerance, as described in greater detail herein.